1. Field of the Invention
The present invention relates to a method for adjusting a developing device including a magnetic roll for holding a two-component developer composed of a carrier and toner filled in a developing tank in a brush state, and a developing roll for developing an electrostatic latent image formed in an image carrier by holding toner conveyed from the magnetic roll, in which there is provided a bias voltage adjustment step for adjusting a conveyance amount of toner from the magnetic roll to the developing roll by adjusting bias voltages applied to the magnetic roll and the developing roll respectively.
2. Description of the Related Art
As a developing device incorporated in an image forming apparatus of an electrophotographic system, Japanese Unexamined Patent Publication No. 2005-55841 proposes a developing method as follows with objects of constantly stabilizing a toner layer thickness on a developing roller over a long period of time without complicating a developing device, and maintaining a stable image without concentration changes.
In the developing method, elicited on an image carrier is a toner layer thickness detecting pattern arranged with a solid pattern using a toner layer formed in the second round from the start of applying a developing bias to a developing roll and a halftone pattern using a toner layer formed during the subsequent one rotation, and a toner concentration of the toner layer thickness detecting pattern on the image carrier or the toner layer thickness detecting pattern transferred to a transfer body is detected by a concentration sensor so as to control a differential voltage of bias voltages between a magnetic roll and a developing roll, i.e. a conveyance bias voltage, on the basis of a detected toner concentration.
However, in the above stated conventional technique, an adjustment is made on condition that an electrification amount of toner filled in a developing tank is controlled within a range of an ideal electrification amount, causing a problem that an adjustment cannot be made appropriately if an electrification amount of the toner is changed.
For example, a concentration of a halftone pattern is detected to set a conveyance bias voltage on the basis of a detected result, which generates leakage between the developing roll and the magnetic roll depending on an electrification amount of toner due to an adjusted conveyance bias value which was made too high, causing occasional image defects such as a black point.
Moreover, a narrowed interval between the developing roll and the magnetic roll causes a conveyance problem of toner, and there is a possibility of generating toner scattering and developer leakages. Oppositely, if the conveyance bias voltage is made too low, it is impossible for a toner supply amount to catch up with a toner consumption amount in the developing roll, and there is a possibility that a developing ghost is generated to have an effect of a previous development history on a subsequent development.